I suspect Niamtic have found all the low hanging fruit, thus no more Seer badges.

It is also possible that Google has refined a "find local points of interest" algorithm which is based on where people stop for longer than might be indicated by traffic lights, pedestrian crossings or other signifiers. Then there is metadata from images (crawl places like Flickr and SmugMug for example).

Add to that the cost of having people review portal submissions and you can invent reasons why Google would want to let Niantic go.